Genetics vs. Environment

Their Role in Intelligence

Nature vs. Nurture in IQ

• “Is a person’s intelligence due more to genes or to environment?”

– Both genes & intelligence crucial for any trait

– Environmental factors determines which genes get switched on.

– Example of nutrition effecting you reaching your genetic potential in height.

Heredity and Environment

• Heritability– The variation within a group caused by genetic factors– Thought to be about 50%

• Environment– Degree to which variation is due to environmental

rather than genetic differences– Environmental Factors can be early intervention,

schooling and family life

Twin Studies & Family Influence Thomas Bouchard & the Minnesota

Twins studies • If trait genetic:

– closely related more similar than less closely related

• Many close relatives share environments too

• Types of studies to separate effects– monozygotic twins reared together

– monozygotic twins reared apart

– siblings/dizygotic reared together

– siblings/dizygotic reared apart

– adoptive siblings reared together

Twin Studies

Data Analysis• The data show that both genetics and environment have an

effect on IQ scores. • The more closely two individuals are related genetically,

the more similar their IQ scores: – Identical twins reared together are more alike than are fraternal

twins reared together.

• Also show the importance of environmental influences: – Identical twins reared together are more alike than are identical

twins reared apart– siblings who are reared together are more alike than are siblings

reared in different homes.

Both genetics and environment have an effect on IQ scores

Adoption StudiesChildren's verbal ability scores became modestly more

like their biological parents' scores.

(Adapted from Plomin & DeFries, 1998.)

Group Differences in Intelligence Test

Scores

Group Differences in Testing• A number of studies show scoring

differences between different racial, ethnic, and gender groups.

• Are these differences due to nature or to nurture? – Studies suggest environment is playing a

heavy role.• Heredity and environment interact to

produce intelligence in individuals.

Why do Japanese and Chinese students perform better than Americans on intelligence and achievement tests?

• They score the same before school but once school starts they begin to show more gains. This due to:– Spending more time in school – More time doing homework – Pressure and support from parents to

achieve academically

Group DifferencesAustralian aborigine & white scores on IQ tests

• Play “Intelligence and Culture” (4:02) Module #4 from The Brain: Teaching Modules (2nd edition).

• Are the differences in scores the result of environment or genes?

• Which theory of intelligence does this example prove? Gardner’s Multiple Intelligences or Spearman’s g factor?

Racial Difference in IQ• Racial difference in average IQ among different racial groups can be measured

• More variation in IQ scores within a particular group than between groups

• Stereotype Threat – When members of a particular group fear they are being thought of or evaluated according to a negative stereotype, they fear that they may confirm the stereotype leading to self-doubt and anxiety resulting in poor performance. (see Box 7.4 on pg. 311)

Effects of Discrimination on Intelligence

• Burakumin people of Japan are severely discriminated against.

• Despite being genetically similar to Japanese people…

• Their IQ scores are 10 to 15 points lower than “normal” Japanese people.

Seed Analogy: Argument for Environment

Within and Between Group Differences

• Each corn field planted from same package of genetically diverse seeds

• One field is quite fertile, the other is not• Within each field, differences due to genetics• Between each field, differences due to environment

(fertility)• Unless the environments of two racial groups are

totally identical, it is impossible to estimate the overall genetic differences between the two groups.

Are IQ Tests Culturally Biased?• If IQ test reflects white, middle-class cultural

knowledge and values it could result in poor scores by minorities.

• Test will favor the people from the culture in which it was developed.

• Factors like test-taking behavior, motivation, attitudes towards the test and previous experiences with tests can affect performance.

• Try some culturally biased tests yourself:– Robert Williams IQ Test & the Dove Counterbalance Test

– Chitling Intelligence Test